The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
The complete removal of adhesive and other residues from the electronic circuit assemblies also pose a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which must be removed include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux and adhesive residues and the like from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
For example, U.S. Pat. No. 3,886,099 discloses a water soluble aqueous amine cleaner to remove rosin flux from electronic circuit assemblies. The cleaner includes an aqueous solution of an aliphatic amine which reacts with the rosin flux to form a soap, a glycol ether used as a flux solvent and an organic acid which serves as a buffer to reduce and maintain the alkalinity of the solution at a pH of between about 10.5 and 11.0. While the patent states that the wash water may be discharged into a normal sewage system because the components are biodegradable, it is unlikely that at the present time the introduction of organics such as rosin fluxes, amines, glycol ethers and acids would be acceptable additions to the environment. In particular, in the disposal of industrial cleaning solutions, municipalities often mandate levels of impurities which are allowable in sewage effluents. Thus, the regulations frequently mandate maximum permitted pH, maximum allowable heavy metals and maximum allowable organics measured as BOD or COD. Accordingly, sewering the effluents from a process for cleaning electronic circuit assemblies with an aqueous amine solution would not be permissible.
On the other hand, treating the wash effluents obtained from the washing of electronic circuit assemblies with aqueous amine solutions by conventional activated carbon treatment and cation exchange would likely capture the amine which is reacted with the rosin to form a soap. The wash water effluent after treatment would include the glycol ethers which would likely pass through the carbon treatment. The amine therefore would have to be continuously replenished if the effluent is to be recycled to the wash cycle of the cleaning process. Likewise, even after carbon and ion exchange treatment, the aqueous effluent would still contain the glycol ether which could not likely be sewered for the reasons stated above.
Accordingly, a treatment process is needed to remove the contaminants from the wash water effluents obtained during the washing of electronic circuit assemblies with aqueous amine solutions so as to provide sufficient removal of the contaminants washed from the electronic circuit assemblies and to retain the active components of the aqueous amine cleaning solution in the effluent, such that the treated effluent can be recycled to the cleaning process without the need for substantial replenishment of the active ingredients to the cleaning solution.